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topic 5- energy changes (exo and endo reactions (exothermic- heat is given…
topic 5- energy changes
exo and endo reactions
energy is moved around in chemical reactions
chemicals store a certain amount of energy - different chemicals store different amounts
if products store more energy than reactants then they must have taken in energy from surroundings (endo)
if they store less then the excess energy was transferred to surroundings
overall amount of energy doesn't change. energy is conserved in reactions
exothermic- heat is given out
an exo reaction is one that transfers energy to the surroundings usually by heating. shown by an INCREASE in temp
exo reaction e.g. combustion (gives out a lot of energy)
e.g. neutralization (acid +alkali) = exo
e.g. oxidation
everyday uses
hand warmers - oxidation of iron in air (with a salt solution catalyst) to release energy
self heating cans of hot drinks
endothermic- heat is taken in
takes in energy from surroundings. shown by DECREASE in temp
endo reactions are less common than exo
e.g. citric acid + sodium hydrogencarbonate
e.g. thermal decomposition- heating calcium carbonate causes it to break down into calcium oxide and carbon dioxide.
everyday uses
sports injury packs, instant cooler
more exo and endo reactions
energy transfer can be measure
(check practical rev cards)
polystyrene cup and cotton wool give more insulation and lid is used to stop evaporation
reaction profiles show energy changes
exo
reactants are at
higher
energy level than products
difference in height between reactants and products represents the overall energy change
the initial rise in energy is the activation energy
activation energy= minimum amount of energy the reactants need to collide with eachother and react. the greater the activation energy the more energy needed can be supplied by heating
endo
reactants are at
lower
energy level than products
the difference in height represents the energy taken in
bond energies
energy must always be supplied to break bonds
during a chemical reaction old bonds are broken and new bonds are formed
energy must be supplied to break existing bonds- endo
energy must be released when new bonds are formed- exo
in exo the energy released by forming bonds is greater than the energy used to break them. opposite for endo
bond energy calculations
every chemical bond has a specific bond energy associated with it
use these to calculate overall energy change in a reaction
sum of energy needed to break bonds - the energy released when new bonds are formed in the products
cells and batteries
chemical reactions in a cell produce electrcity
an electrochemical cell is a basic system made up of two different electrodes in contact with electrolyte
the two electrodes must be able to conduct electricity so are usually metals
the electrolyte is a liquid that contains ions which react with the electrodes
the chemical reactions between the electrodes and electrolyte set up a charge difference between electrodes
if the electrodes are connected by a wire charge is able to flow and energy is produced can be measured with voltmeter
the voltage of a cell depends on many factors
different metals react differently with same electrolyte - this is what causes charge difference or voltage in the cell
the bigger the difference in reactivity in the electrodes the bigger the voltage of the cell
the electrolyte used in a cell will also effect the size of the voltage since the different ions in solution will react differently with the metal electrodes
a battery is formed by connecting two or more cells together in series. the voltages of the cells in the battery are combined so there is a bigger voltage overall
in non rechargeable batteries reactants get used up
in some cells the chemical reactions that happen at the electrodes are irreversible
over time the reacting particles get used up and turned into products of the reaction
once one reactant is used up reaction can no longer happen so no electricity is produced
products cant be turned back into reactants so battery cannot be recharged
in rechargeable batteries the reaction can be reversed by connecting it to an external electric current
fuel cells
fuel cells use fuel and oxygen to produce electrical energy
a fuel cell is an electrical cell that's supplied with a fuel and oxygen and uses energy from the reaction between them to produce electricity efficiently
when fuel enters cell it becomes oxidized and sets up a pd within the cell
there are a few different types of fuel cells using different types of fuel and electrolyte. e.g. hydrogen- oxygen fuel cell
hydrogen- oxygen fuel cells involve a
redox
reaction
the electrolyte is often a solution of potassium hydroxide
hydrogen goes to anode compartment and oxygen goes to cathode compartment
at the anode hydrogen loses electrons to produce H+ ions (oxidation)- H2---> 2H+ + 2e-
H+ ions in the electrolyte move to cathode
at the cathode oxygen gains electrons from the cathode and reacts with H+ ions (from electrolyte) to make water (reduction) - O2 + 4H+ +4e- ---> 2H2O
the electrons flow through external circuit from anode to cathode- electric current
the overall reaction is 2H2 + O2 ----> 2H2O
hydrogen- oxygen fuel cells could be used in vechiles
conventional fuels (petrol) are finite and are very polluting. vehicles that use electric energy are becoming more popular
pros
don't produce as many pollutants as other fuels (greenhouse gases)
only by product is water and heat
electric vehicles
pros
don't produce much pollutants
batteries are rechargeable
cons
batteries are more polluting to dispose of
batteries made from highly toxic metal compounds
limit to how many times they can be recharged before they have to be replaced
batteries are more expensive to make than fuel cells
batteries store less energy than fuel cells
cons
hydrogen needs lots of space to store
specially adapted car to burn oxygen